Article pubs.acs.org/JPCC NO Adsorption on Copper Phthalocyanine Functionalized Graphite † ‡ § ‡ Jun Hong Park, , Pabitra Choudhury, and Andrew C. Kummel*, † ‡ Materials Science & Engineering Program and Departments of Chemistry & Biochemistry, University of California, San Diego and La Jolla, California 92093, United States § Department of Chemical Engineering, New Mexico Tech, Socorro, New Mexico 87801, United States *S Supporting Information ABSTRACT: NO dosed on a CuPc monolayer deposited on Au(111) and HOPG is observed by scanning tunneling microscopy. After dosing NO with a supersonic molecular beam source onto CuPc/Au(111), about 7% of CuPc molecules form chemisorbates with NO. Conversely, after dosing onto CuPc/HOPG, only about 0.1% CuPc molecules form chemisorbates with NO, even though the reaction sites appear nearly identical. DFT calculations were employed to elucidate the mechanism which causes the >10× difference in saturation coverage between NO/CuPc/Au(111) and NO/ CuPc/HOPG. DFT calculations show NO chemisorption with CuPc/Au(111) induces only negligible perturbation in the density of states (DOS) in Au(111) due to large density of states on Au. Conversely, for NO/CuPc/HOPG, there is a large decrease of DOS in graphene around 1 eV due to NO chemisorption on CuPc/graphene consistent with negative charge transfer from graphene to NO. This DOS perturbation of graphene results in decreased binding energy of NO chemisorption in secondary NO sites, consistent with low saturation coverage. The results suggest that although the saturation coverage of NO chemisorbates is low on CuPc/graphene, the DOS of graphene can be altered by low coverages of adsorbates even onto weakly interacting molecules which chemically functionalize the graphene surface. ■ INTRODUCTION structure at high temperature and only a few acids are able to π Graphene, a 2D semiconductor material, consists of sp2 bonded decompose MPcs. MPc molecules have delocalized -electrons, carbon atoms in a honeycomb lattice forming a single layer.1 enabling them to act as electron donors, thereby providing a low activation energy for formation of charge transfer Graphene has high carrier mobility as well as a chemically and 17,28,29 electronically passivated surface, thereby rendering graphene to complexes with oxidizing analytes in chemical sensing. − be a promising electronic material for novel devices.2 5 MPc detection of chemical analytes is induced primarily by Graphene also has potential for chemical sensing platforms, charge transfer reactions between the central metal ion and 6,7 analytes; therefore, in a simplified model, the analytes act as due to its atomically thin body. The atomically thin body of “ ” 30,31 fi fi graphene allows each carbon atom to directly interact with MPc dopants : Pure MPcs lms of organic thin lm ambient analytes, thereby increasing chemical sensitivity. transistor (OTFT) in vacuum have a Fermi level in the middle of the band gap. However, when MPcs films are exposed to However, the surface of graphene is chemically inert, thereby 17,30−32 requiring modification for use in electronic and sensor oxidative agents, MPc become p-type semiconductors. − fi devices.8 10 One common approach is oxidation of graphene The induced p-type conductivity of MPc lms has been with chemical reduction, but this oxidation also involves modeled as formation of charge transfer complexes on the 11−13 ff metal centers with oxidative analytes. Oxidative analytes (such introduction of defects. An e ective strategy to modify the 20,31 chemical sensitivity of graphene is functionalization with other as O2), which are electron acceptors, chemisorb on MPc molecules, forming superoxide adducts consisting of oxidized materials without perturbing the graphene electronic structure. + 2− For example, a self-assembly layer by organic molecules can be MPc and O species. During this reaction, electrons are effective for fabrication of graphene based chemical or bio transferred from MPc to analytes. Simultaneously, holes are 14−16 injected into the MPc molecules to form positively charged sensors. + Metal phthalocyanines (MPc) may enhance the sensing MPc . The injection of these holes moves the HOMO edge fi 17 performance of graphene chemical sensors via functionalization. toward the Fermi level forming a p-type MPc lm. Due to − MPc have been widely studied as chemical sensors.17 22 The MPc films can be deposited to form well-ordered layers on Received: January 8, 2014 inorganic substrates, including graphene or graphite surfa- Revised: April 19, 2014 − ces.23 27 Moreover, MPcs can maintain their molecular Published: April 22, 2014 © 2014 American Chemical Society 10076 dx.doi.org/10.1021/jp5002258 | J. Phys. Chem. C 2014, 118, 10076−10082 The Journal of Physical Chemistry C Article Figure 1. Submolecular resolution empty state STM images of a CuPc monolayer surface dosed with NO on Au(111) and HOPG at 150 K. Diluted s NO was dosed by MBS, and nozzle was held at 300 K. Empty state STM images were recorded with V = +2.0 V, It = 20 pA. The pulsing of the MBS was 30 Hz with 100 μs opening time. Scale bars indicate 2 nm. The reacted sites are denoted by circles. (a) Submolecular resolution of CuPc chemisorbed with single NO molecule on Au(111). The center of CuPc is modified into bright spot. (b) Submolecular resolution of CuPc chemisorbed with single NO molecule on HOPG. The center of CuPc is modified into bright spot. (c) Chemisorption model of NO with CuPc. Figure 2. Scanning tunneling spectroscopy (STS) dI/dV curves of unreacted and reacted CuPc molecules. All spectra recorded at the center of the CuPc and were swept from −2 to 2 V. (a) STS dI/dV spectra of NO/CuPc/Au(111). (b) STS dI/dV spectra of NO/CuPc/HOPG. The black spectra indicate STS from unreacted CuPc molecules, while red spectra show STS from CuPc molecules with chemisorbed NO. After NO chemisorption on CuPc molecules, the EF was shifted toward the VB (HOMO) in both NO/CuPc/Au(111) and NO/CuPc/HOPG. these properties and MPc adsorption on graphene not obtained in STM imaging. The Highly Ordered Pyrolytic perturbing its atomic structure, MPc molecules are suitable to Graphite (HOPG) substrate was cleaved in air before being functionalize graphene for chemical sensing. loaded into the vacuum chamber. Afterward, the cleaved The present study employs molecular scale imaging and HOPG sample was annealed at 823 K for 5 h to obtain large, tunneling spectroscopy to investigate NO adsorption on copper flat and clean areas as shown in the STM imaging. phthalocyanine (CuPc) monolayers deposited on highly The CuPc was purchased from Sigma-Aldrich and purified by oriented pyrolytic graphite (HOPG). The HOPG surface has multiple sublimation cycles. The CuPc monolayer sample was similar electronic properties to graphene, including a nearly deposited as thick overlayers of CuPc on clean Au(111) and inert surface and a zero band gap. NO also was dosed onto HOPG surfaces by organic molecular beam epitaxy with a CuPc/Au(111) for comparison with NO/CuPc/HOPG. Using differentially pumped effusion cell (Eberl MBE- Komponent- STM, the dependence of NO absorption on the MPc support en), while the sample was in the UHV preparation chamber, as (HOPG vs Au(111)) was elucidated. To understand the shown in Suppoting Information, SI, Figure S1. During electronic changes of the HOPG surface induced by generation deposition, both the Au(111) and HOPG were held at 373 of NO/CuPc/HOPG complexes, DFT calculations were K. Subsequently, the CuPc multilayers on Au(111) were heated performed. to 623 K for 4 min to form a flat-lying monolayer of CuPc on the Au(111) observed by in situ STM, because the CuPc/CuPc ■ EXPERIMENTAL SECTION interactions in multilayers are weaker than the CuPc/Au(111) 33 All experiments were performed in a commercial multichamber surface interaction. Conversely, the CuPc multilayer on Omicron ultrahigh vacuum (UHV) system with a base pressure HOPG was annealed at 473 K for 6 min surface to form a flat- below 1 × 10−10 Torr. Clean Au(111) and HOPG surfaces lying monolayer of CuPc on the HOPG observed by in situ were prepared as substrates for CuPc deposition. The surface of STM. This lower annealing temperature on HOPG results from a single crystal Au(111) was cleaned by multiple sputtering the weaker interaction CuPc with HOPG than with Au(111). cycles with a 0.5 to 1 kV of Ar+ ion beam (RBD instruments) NO was dosed on CuPc monolayer in the UHV chamber at with an Ar background pressure of 6 × 10−5 Torr at 300 K 150 K with use of a pulsed supersonic molecular beam source surface temperature. After sputtering, the Au(111) sample was (General Valve Series 1), which was differentially pumped using annealed at 773 K for 30 min. This sputtering and annealing a three chamber differentially pumped source with turbomo- cycle was repeated until an atomically flat Au(111) surface was lecular pumps (TMP). The NO analyte was diluted in He (He: 10077 dx.doi.org/10.1021/jp5002258 | J. Phys. Chem. C 2014, 118, 10076−10082 The Journal of Physical Chemistry C Article 95%; NO: 5%). The low surface temperature was employed so tilting of NO binding consistent with the bound NO that the chemisorbate would be stable on the surface for STM continuously rotating on Cu ion because NO has a single Imaging. The pulsed molecular beam operated at 30 Hz with a bond to Cu. A schematic binding configuration of NO with 100 μs opening time. Note this the supersonic molecular beam CuPc molecules is presented in Figure 1(c). source will produce a nearly monoenergetic beam of NO with Scanning tunneling spectroscopy (STS) was employed to an approximate translational energy of 0.33 eV, so it slightly investigate the changes in the electronic structure of CuPc/ favors direct chemisorption over precursor chemisorption Au(111) and CuPc/HOPG due to NO chemisorption, as compared to a thermal gas source at 300 K.34 Before flowing, shown in Figure 2.